1. Field of the Invention
The present invention relates to a liquid crystal display device, and, more particularly, to a method and an apparatus for driving a liquid crystal display device that reduce the number of frame memories.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device controls light transmittance of liquid crystal cells in accordance with data signals applied thereto, to thereby display an image. In particular, an active matrix type LCD device includes a switching device for each cell and has various applications, such as a monitor for a computer, office equipment, and a cellular phone, because of their high quality image, lightness, thin thickness, compact size, and low power consumption. A thin film transistor (TFT) is generally employed as the switching device for the active matrix type LCD device.
As can be seen from the following Formulas 1 and 2, the liquid crystal display device has a disadvantage that its response time is slow due to its properties, such as the unique viscosity and elasticity of a liquid crystal material.
                              τ          r                ∝                              γ            ⁢                                                  ⁢                          d              2                                            Δɛ            |                                          V                a                2                            -                              V                F                2                                      |                                              Formula        ⁢                                  ⁢        1            In particular, τr represents a rising time when a voltage is applied to the liquid crystal material; Va represents an applied voltage; VF represents a Frederick transition voltage by which liquid crystal molecules begin to make a tilt motion; d represents a cell gap of the liquid crystal cell; and γ represents a rotational viscosity of the liquid crystal molecules.
                              τ          f                ∝                              γ            ⁢                                                  ⁢                          d              2                                K                                    Formula        ⁢                                  ⁢        2            In addition, τf represents a falling time at which liquid crystal material is restored to its initial position by an elastic restoration force after the voltage applied to the liquid crystal material was turned off; and K represents a unique elastic coefficient of the liquid crystal material.
A response speed of the liquid crystal material in a twisted nematic (TN) mode, which is a liquid crystal mode having been most widely used in the liquid crystal display device up to now, can be differentiated in accordance with the physical properties and the cell gap of the liquid crystal material, but generally its rising time is about 20 ms˜80 ms and its falling time is about 20 ms˜30 ms. The response speed of such a liquid crystal material is longer than one frame interval (e.g., 16.67 ms in the case of the NTSC system). For this reason, a voltage charged in the liquid crystal cell is progressed into the next frame before it arrives at a desired voltage as shown in FIG. 1, thereby causing a motion-blurring phenomenon in which the screen gets blurred in the moving picture.
FIG. 1 is a waveform illustrating a change of brightness according to a data in a liquid crystal display device according to the related art. In FIG. 1, when a data VD is changed from one level to another level, a display brightness BL corresponding to such a level change fails to reach a desired brightness and hence fails to express desired color and brightness. As a result, the liquid crystal display device has a motion-blurring phenomenon appearing in the moving picture, and has a poor picture quality due to a deterioration of contrast ratio.
In order to resolve the slow response speed of the liquid crystal display device, U.S. Pat. No. 5,495,265 and PCT international publication No. WO 99/05567 have introduced a scheme of modulating a data depending upon whether or not the data is changed by using a look-up table (hereinafter referred to as “high-speed driving method”), as shown in FIG. 2.
FIG. 2 is a waveform illustrating an example of the change of brightness according to a data modulation in a high-speed driving system according to the related art. In FIG. 2, the high-speed driving method modulates an input data VD to generate a predetermined modulated data MVD and applies the modulated data MVD to a liquid crystal cell, thereby obtaining a desired brightness MBL. The high-speed driving method enlarges a value of |Va 2−VF2| in Formula 1 on the basis of a change of the data so that it can obtain a desired brightness MBL in correspondence with a brightness value of the input data VD within one frame interval. In particular, a data at the previous frame is compared with a data at the current frame. If a change between the data exists, the data at the current frame is modulated to a predetermined modulated data. Accordingly, the liquid crystal display device adopting the high-speed driving method compensates a slow response speed of the liquid crystal material, to thereby alleviate the motion-blurring phenomenon in the moving picture.
FIG. 3 is a block diagram illustrating an example of a high-speed driving apparatus according to the related art. In FIG. 3, the high-speed driving apparatus includes first and second frame memories 43a and 43b for storing data DataIn supplied from a data bus 42, and a modulator 44 for modulating the data. The first and the second frame memories 43a and 43b alternately store data for each frame unit in accordance with a pixel clock and then alternately output the stored data to supply a previous frame data, i.e., the (n−1)th frame data Fn−1 to the modulator 44.
The modulator 44 compares an nth frame data Fn from the data bus 42 with an (n−1)th frame data Fn−1 from the first and second frame memories 43a and 43b, and then selects a modulated data MRGB corresponding to the compared result from a look-up table. The look-up table may be as shown in Table 1 to modulate the data and is stored in a read only memory (ROM).
TABLE 101234567891011121314150023456791012131415151515101345678101213141515151520024567810121314151515153001356781011131415151515400134678911121314151515500123578911121314151515600123468910121314151515700123457910111314151515800123456810111214151515900123456791112131415151000123456781012131415151100123456789111314151512001234567891012141515130012334567810111315151400123345678911121415150001233456789111315
In Table 1, the leftmost column represents the data at the previous frame Fn−1 and the uppermost row represents the data at the current frame Fn.
During the nth frame interval, as indicated by a solid line in FIG. 3, the nth frame data Fn is stored in the first frame memory 43a in accordance with the same pixel clock and, simultaneously, is supplied to the modulator 44. In addition, during the nth frame interval, the second frame memory 43b supplies the (n−1)th frame data Fn−1 to the modulator 44.
Then, during the (n+1)th frame interval, as indicated by a dotted line in FIG. 3, the (n+1)th frame data Fn+1 is stored in the second frame memory 43b in accordance with the same pixel clock and, simultaneously, is supplied to the modulator 44. In addition, during the (n+1)th frame period, the first frame memory 43b supplies the nth frame data Fn to the modulator 44.
As described above, the high-speed driving apparatus requires two frame memories 43a and 43b in order to alternately supply the previous frame data to the modulator 44. Since the frame memories increase fabrication costs, it is necessary to provide a scheme capable of reducing the number of the frame memories or a capacity of the memory.